pgsql: Fix typo in previous release as reported by Itagaki Takahiro, but

Log Message:
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Fix typo in previous release as reported by Itagaki Takahiro, but missed
by me.

Modified Files:
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pgsql/src/backend/utils/mb:
mbutils.c (r1.90 -> r1.91)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/mb/mbutils.c?r1=1.90&r2=1.91)